Rotating electrical machines typically comprise a cylindrical rotor mounted to rotate around or within an adjacent stator. A rotating electrical machine may thus have an internal substantially cylindrical rotor mounted within an external stator or an outer cylindrical rotor mounted around an internal stator. Some types of rotating electrical machine, such as a double output generator, have both internal and external substantially cylindrical stators with a rotor located between them.
The construction of rotors for low speed high torque rotating electrical machines can be problematic, and this is especially the case for rotating electrical machines that have internal and external cylindrical stators. For example, in a typical permanent magnet generator employed in a direct drive wind turbine, the rotor has a diameter of the order of several meters, whereas the air gaps between the rotor and the adjacent internal and external stators are typically of the order of several millimeters.
Because the air gaps between the rotor and internal and external cylindrical stators are so small compared to the diameter of the rotor, the rotor and the internal and external cylindrical stators must be manufactured with a high degree of roundness and then carefully aligned with each other to provide uniform air gaps. It can be difficult to ensure concentricity and axial alignment of the cylindrical rotor and adjacent cylindrical stators and the manufacture and assembly of rotating electrical machines having internal and external cylindrical stators can thus be complex.
There is, therefore, a need for an electrical machine having an improved rotor construction which overcomes the difficulties mentioned above.